1. Field of Invention
The present invention relates to a microwave sensor that is an active sensor using electromagnetic waves whose frequency is lower than that of visible light. In particular, the present invention relates to a microwave sensor and a mutual interference preventing system between microwave sensors in which an influence of mutual interference between their radio waves can be suppressed in the case where a plurality of microwave sensors are arranged close to each other.
2. Conventional Art
Conventionally, as one crime prevention device, microwave sensors are known in which microwaves are emitted toward a detection area, and when a human figure is present in the detection area, the human figure (intruder) is detected by receiving the reflected waves (microwaves modulated due to the Doppler effect) from the human figure.
Such a microwave sensor is provided with an antenna for emitting and receiving microwaves. Microwaves are emitted from the antenna toward a detection area, and when a human figure is present in the detection area, the reflected waves from the human figure with the frequency modulated due to the Doppler effect are received by the antenna. More specifically, in this case, the microwaves received by the antenna are modulated with respect to the frequency of the microwaves emitted from the antenna, so that the waveforms of an output signal from the microwave sensor is changed, and thus a human figure detection signal is emitted from the microwave sensor.
Generally, this type of microwave sensor is used in combination with a passive infrared sensor (PIR sensor) in which an infrared ray from a human figure in a detection area is received, and the intruder is detected based on a temperature difference between the human figure and its surroundings (see JP H11-39574A, for example). More specifically, the detection area of the microwave sensor and the detection area of the passive infrared sensor are overlapped, and the AND of their detection outputs is taken so as to supplement weaknesses of the two sensors, so that the reliability of human figure detection is enhanced.
When a plurality of such microwave sensors are arranged in the same space or one in each adjacent space, radio waves emitted from the microwave sensors may interfere each other. Normally, the antennas of microwave sensors are arranged to extend vertically in the state where sensors are installed. When a pair of the thus configured sensors are arranged, for example, on wall surfaces opposed to each other in the same room, the planes of polarization of the antennas of the microwave sensors overlap each other on the same plane, and thus their radio waves interfere with each other. Consequently, a noise is mixed in the waveforms of output signals from the microwave sensors, and thus a normal operation may be impaired. Furthermore, even when the microwave sensors are arranged one in each adjacent room, if the wall surfaces on which the microwave sensors are arranged are opposed to each other, their radio waves interfere each other in a similar manner to the above because microwaves are transmitted through walls, and thus a normal operation may be impaired.
FIG. 4 is a block diagram showing a circuit configuration of such a conventional microwave sensor 100.
As shown in FIG. 4, the microwave sensor 100 is provided with an oscillation power source 26 for oscillating microwaves, a transmitting antenna 22 for transmitting the microwaves oscillated by the oscillation power source 26 toward a detection area, a receiving antenna 21 for receiving the reflected waves of the microwaves reflected by a human figure or the like, a mixer 23 for mixing the microwaves received by the receiving antenna 21 and the voltage waveforms of the oscillation power source 26 and outputting the result, an IF amplifier 25 for amplifying the output of the mixer 23, a microprocessor 110 for controlling the entire microwave sensor 100, and an oscillation circuit 11 for supplying a clock signal CLK to the microprocessor 110. It should be noted that for the oscillation circuit 11, for example, a ceramic oscillator or a crystal oscillator can be used, but the oscillator is not limited to these.
Furthermore, a switch 24a is inserted between the mixer 23 and the IF amplifier 25, and a switch 24b is inserted between the transmitting antenna 22 and the oscillation power source 26. The switches 24a and 24b can switch an electrical connection state in response to an external signal, and are connected so as to be switchable in synchronization.
The microprocessor 110 has a switching control portion 10a for outputting a switching control signal S0 that controls switching of the switches 24a and 24b, a timer 10b for determining the cycle of the switching control signal S0 that is output from the switching control portion 10a, and a time setting portion 10c for setting a detection cycle (for example, 250 μs) for the timer 10b. For the ON time of the switching control signal S0 in each cycle, a necessary time can be ensured by using, for example, another timer (not shown) or a software timer.
The microprocessor 110 generates a system clock by dividing the clock signal CLK supplied from the oscillation circuit 11, and operates each portion of the microprocessor 110 based on the system clock. Since the timer 10b also operates based on the system clock, the accuracy of time of the timer 10b depends on the accuracy of the system clock or the clock signal CLK of, the oscillation circuit 11 from which the system clock is generated.
When the switching control signal S0 that is output from the switching control portion 10a is ON, both of the switches 24a and 24b are switched to be electrically connected, and thus the microwave sensor 100 performs an operation of detecting a human figure or the like. More specifically, microwaves are transmitted from the transmitting antenna 22 toward a detection area, and when a human figure or the like is present in the detection area, the reflected waves from the human figure with the frequency modulated due to the Doppler effect are received by the receiving antenna 21. The received reflected waves are mixed with the voltage waveforms of the oscillation power source 26 by the mixer 23 and amplified by the IF amplifier 25, and then an IF output signal IFout0 from the IF amplifier 25 is obtained as a human figure detection signal output from the microwave sensor 100. When there is no human figure or the like in the detection area, reflected waves whose frequency is modulated are not received by the receiving antenna 21. Therefore, the IF frequency of the IF output signal IFout0 from the IF amplifier 25 is “0,” and thus a human Figure detection signal is not output from the microwave sensor 100.
On the other hand, when the switching control signal S0 that is output from the switching control portion 10a is OFF, both of the switches 24a and 24b are switched to be electrically disconnected, and thus the microwave sensor 100 does not perform an operation of detecting a human figure or the like.
FIGS. 5(a) and 5(b) are examples of a time chart for comparing switching control signals S0 when two conventional microwave sensors 100 are used. FIG. 5(a) shows the switching control signal S0 of a first microwave sensor, and FIG. 5(b) shows the switching control signal S0 of a second microwave sensor.
As shown in FIGS. 5(a) and 5(b), these microwave sensors 100 perform operations of detecting a human figure or the like intermittently at a predetermined detection cycle. The first microwave sensor 100 has a cycle T01a, and performs a detection operation during a time T02a during which the switching control signal S0 is ON, in each cycle. The second microwave sensor 100 has a cycle T01b, and performs a detection operation during a time T02b during which the switching control signal S0 is ON, in each cycle. The cycle of the switching control signal S0 may be set to, for example, 250 μs, and the ON time may be set to, for example, 50 μs, but the time setting is not limited to this.
When the two microwave sensors 100 are used close to each other, for example, if the timings at which the switching control signals S0 of the first microwave sensor and the second microwave sensor are ON are sufficiently apart from each other on the time axis, it can be said that their radio waves do not interfere with each other and thus a normal operation is not impaired.
Furthermore, when the cycle T01a and the cycle T01b of the switching control signals S0 of the microwave sensors 100 are completely identical to each other, the timings at which the switching control signals S0 are ON are always kept at the same distance on the time axis from each other. Therefore, unless the timings at which the switching control signals S0 are ON overlap each other accidentally from the beginning, their radio waves do not interfere with each other.
FIGS. 6(a) and 6(b) are examples of a time chart for comparing switching control signals S0 at a different time point from that of FIGS. 5(a) and 5(b), when two conventional microwave sensors 100 are used in a similar manner. FIG. 6(a) shows the switching control signal S0 of a first microwave sensor, and FIG. 6(b) shows the switching control signal S0 of a second microwave sensor. FIG. 7 is an example of a waveform of an IF output signal IFout0 from the IF amplifier 25 of one of the microwave sensors 100 in this case.
As described above, the cycles of the switching control signals S0 are determined by the timers 10b of the microprocessors 110, and the accuracy of time of the timers 10b depends on the accuracy of the system clocks or the clock signals CLK of the oscillation circuits 11 from which the system clocks are generated. Although the accuracy of frequency of, for example, a ceramic oscillator or a crystal oscillator used for the oscillation circuits 11 is high, there is a slight error with respect to a reference frequency, and this error is different from oscillator to oscillator. More specifically, the cycles of the switching control signals S0 are slightly different for each microwave sensor 100 in the strict sense, and the cycle T01a and the cycle T01b of the switching control signals S0 in FIGS. 6(a) and 6(b) are slightly different from each other.
Therefore, a distance on the time axis between the timings at which the switching control signals S0 of the first microwave sensor and the second microwave sensor are ON changes in a long period of time, and the timings at which the switching control signals S0 are ON almost overlap each other in the course of time as shown in FIGS. 6(a) and 6(b). In this state, their radio waves interfere each other, and thus a noise is generated. This state continues for a while, and after a further time has passed, the timings at which the switching control signals S0 are ON do not overlap each other again, and then the same process is repeated cyclically. When the noise caused by such interference between radio waves is referred to as “interference noise,” the interference noise in the IF output signal IFout0 from the IF amplifier 25 of one of the microwave sensors 100 has a waveform, for example, as shown in FIG. 7. In this example, the frequency of the interference noise is about 14 Hz.
Since the interference noise is generated in a certain cycle based on the cycle T01a and the cycle T01b of the switching control signals S0, it is possible to calculate the cycle of the interference noise or a frequency f0 of the interference noise, which is an inverse number of the cycle. When the ratio of a difference between the frequencies of the clock signals CLK of the oscillation circuits 11 of the two microwave sensors 100 is taken as “A,” and the cycle of the switching signals S0 is taken as “T01,” the frequency f0 of the interference noise can be expressed by the following equation.f0=A/T01  (1)
When A=3530 [ppm] and T01=250 [μs] are inserted into Equation 1, f0≈14.1 [Hz] results, which is nearly equal to the frequency of the interference noise shown in FIG. 7.
It should be noted that the ratio “A” of a difference between the frequencies of the clock signals CLK actually can take a value in a range up to about several thousands ppm in the case of, for example, a ceramic oscillator, and takes a different value from oscillator to oscillator. Therefore, the frequency of an interference noise differs based on the combination of two microwave sensors 100.
In the case where the frequency of the interference noise is within the frequency band (for example, 5 to 50 Hz) of a signal that is output when the microwave sensor 100 detects a human figure or the like, the interference noise is amplified by the IF amplifier 25, and is output as a human figure detection signal from the microwave sensor 100.
As one of means for preventing such interference between radio waves, the frequencies of microwaves emitted by microwave sensors are differentiated from each other.
Furthermore, there is also a method in which microwave sensors are electrically connected to each other to use a common synchronizing signal, so that timings of detection operations performed by the microwave sensors do not overlap each other.
Alternatively, microwave sensors have been proposed in which the antennas of the microwave sensors are arranged to be inclined with respect to the vertical direction, so that the planes of polarization of the antennas do not overlap each other on the same plane to prevent the interference (see JP 2002-311154A, for example). The microwave sensors provided with an antenna for emitting microwaves toward a detection area and for receiving the microwaves reflected from the detection area, in which a human figure in the detection area is detected based on the microwaves received by the antenna, is characterized in that the antenna is provided to extend in an oblique direction, not in the vertical direction or the horizontal direction in the state where sensors are installed.
However, when the frequencies of the microwaves emitted by the microwave sensors are differentiated from each other as the above-described conventional technique, there is the problem of the frequency band that can be actually used being often regulated by, for example, national laws and systems. Therefore, a large number of microwave sensors using different frequencies cannot be prepared.
Furthermore, when microwave sensors are electrically connected to each other to use a common synchronizing signal, a wiring work becomes necessary. Thus, not only is the installation work difficult, but also new problems stemming from the wiring may occur (for example, a normal operation of a part of or all microwave sensors is impaired due to contact failure of wires, disconnection of wires or the like).
The method for arranging the antennas of microwave sensors to be inclined with respect to the vertical direction may be difficult to adapt in practice in some installation locations.